EP2003474A2 - Method for structuring the surface of an optical component using laser machining - Google Patents

Abstract

Process for preparation of an optical component for electromagnetic radiation by means of laser machining involving the steps: (1) preparation of a solid body from a material which in the untreated state absorbs laser radiation of the machining wavelength range; (2) laser machining of the solid body in one or more machining steps; and (3) material transformation in which the profile obtained is transformed into a final state transparent for the applied wavelength range.

Description

The invention relates to a method for producing an optical component with which an optical function for electromagnetic radiation of an application wavelength range can be achieved due to its surface structured by a multistage profile, the surface structuring being carried out with the aid of ablative laser processing with laser radiation of a processing wavelength range.

Micromachining with lasers is used for various applications of optics, mechanics, flow technology and electronics. Examples include drilling microholes in circuit boards or inkjet heads, trimming electrical components, stripping wires, or manufacturing medical stents. In the fabrication of finely structured optical components, such as those components required for beam guidance and beam homogenization for these applications, the precision of machining is closely related to the optical absorbance at the corresponding laser wavelength. Only highly absorbent material can be processed with high precision. In particular for generating a certain profile, or a specific structure, by laser ablation is a high absorption capacity of the material to be processed condition. In this case, the desired structure, for example a mask, is produced by removing the material at the irradiated points. On the other hand, optical glasses and other optical materials (crystals) in the optical spectral range, ie visible to ultraviolet (UV) wavelengths, absorb only weakly as intended and therefore can not be processed with high precision with lasers emitting in this spectral range.

Of particular importance is the increasing demand for functional UV optics, for example on diffractive optical elements (phase elements, amplitude elements) for beam shaping and beam homogenization of pulsed UV excimer lasers, which are predestined for micromaterial processing. Since these optics should be transparent, in particular for the typical wavelengths (193 nm, 248 nm) of the excimer lasers, only shorter wavelengths (157 nm), for which the optical material, for example quartz glass, is sufficiently absorbent, are suitable for producing these optics. The processing with such extremely short wavelengths is extremely difficult and expensive. In addition, the application of optics thus produced is always limited to higher application wavelengths than the processing wavelength. Therefore, laser ablation has not yet established itself in optical production, although it has advantages in particular in the production of miniaturized, complex-shaped optics, compared to classical (mechanical grinding and polishing) or lithographic production techniques. Especially in the production of individual pieces or Small series is the cost in terms of cost, time and complexity of the manufacturing process, through the various steps of lithographic processes (coating, exposure, development, etching) high and limited flexibility in the design. In particular, the laser processing can also be easily applied to curved surfaces, in contrast to lithographic methods.

From the DE 100 17 614 A1 For example, there is known a method of fabricating a dielectric reflection mask in which a processing wavelength absorbing layer is disposed prior to patterning laser irradiation of the device, between a substrate and a reflective mask layer system comprising alternating high and low refractive index layers. After patterning ablation of the absorption layer, remnants of the absorption layer that are not removed can remain at the irradiated points, which can undesirably impair the transparency of the mask for the application wavelength. These residues can then be converted to a transparent material in a subsequent material conversion step to enhance mask transmission.

Although the known method has the advantage that in the production of a reflection mask, the restriction of the application wavelength to wavelengths greater than the processing wavelength is canceled, but adversely affects that an optical function of the component only in cooperation with the previously applied Mask layer is realized. The absorption layer, however, is effective only in the preparation of the mask. The method is also suitable only for backside ablation from the substrate side.

For the production of other components, especially with multi-stage profiles, as they are required for many applications, this method is rather unsuitable.

The object of the present invention is therefore to provide a method for producing an optical component for electromagnetic radiation by means of laser processing, in which the application wavelength of the intended application electromagnetic radiation is independent of the processing wavelength of the laser radiation selectable with the on and Multi-stage profiles can be produced on the component, and which is flexibly applicable to various types of components optionally with front and / or back machining.

• bei dem Depositionsschritt eine Absorptionsschicht aus einem Oxid-Material, das in einem unvollständig oxidierten Rohzustand die Laserstrahlung des Bearbeitungs-Wellenlängenbereichs in für die ablatierende Laserbearbeitung ausreichendem Maße und die elektromagnetische Strahlung des Anwendungs-Wellenlängenbereiches in für die Erzielung der optischen Funktion zu hohem Maße absorbiert und das in einem vollständig oxidierten Zustand ein geringeres Maß an Absorption für die elektromagnetische Strahlung des Anwendungs-Wellenlängenbereiches aufweist, aufgebracht wird, und
• bei dem Ablationsschritt die aufgebrachte Absorptionsschicht mit Laserlicht der Bearbeitungswellenlänge bestrahlt und an den bestrahlten Stellen wenigstens über einen Teil der Schichtdicke ablatiert wird,

und dass mindestens einmal ein Materialumwandlungsschritt durch Oxidation durchgeführt wird, bei dem das erzeugte Profil in einen Endzustand, in dem es für die elektromagnetische Strahlung im Anwendungs-Wellenlängenbereich in für die optische Funktion ausreichendem Maße transparent ist, umgewandelt wird.This object is achieved in conjunction with the preamble of claim 1, characterized in that for generating the multi-stage profile, starting from a substrate body transparent to the processing wavelength range, a processing cycle is repeated several times, each consisting of a deposition step and an ablation step, wherein

• in the depositing step, an absorbing layer of an oxide material which, in an incompletely oxidized raw state, absorbs the laser radiation of the machining wavelength range in the laser radiation ablating laser machining and the electromagnetic radiation of the application wavelength range to a high degree for the achievement of the optical function and in a fully oxidized state, a lower level of absorption for the electromagnetic radiation of the Application wavelength range is applied, and
• in the ablation step, the applied absorption layer is irradiated with laser light of the processing wavelength and ablated at the irradiated points over at least part of the layer thickness,

and that a material conversion step is performed at least once by oxidation, wherein the generated profile is converted to a final state in which it is transparent to the electromagnetic radiation in the application wavelength range to a degree sufficient for the optical function.

The method according to the invention provides a process in which the laser processing (surface structuring) of the component for producing the optical function can take place in a sufficiently absorbing state. The mold-finished member is rendered transparent to enable the optical function using a material whose absorption / transmission can be changed over a wide range by material conversion which leaves the mold unchanged.

By using for the absorption layer a material which, in an initial state, absorbs the wavelength of the processing laser and, in a final state, after a material transformation, is transparent in the application wavelength range, an optical component can be used for wavelengths which are independent of the processing wavelength be prepared without an absorption layer must be applied, as the Raw material itself has the absorption property. This has a particularly cost-effective and time-saving. In particular, the processing wavelength may be within the (later) application wavelength range, for example, a processing wavelength and an application wavelength may each be 193nm.

Since the suitable starting materials usually show a continuously decreasing absorption behavior of short to long wavelengths, and thus have a correspondingly continuously decreasing laser machinability and a continuously increasing applicability as an optical element, intermediate states are also possible in the manufacture of the component, which this gradual absorption / - take into account transmission changes. For example, the source material may be highly absorbent for a first wavelength in the processing wavelength region and moderately absorbent for a second wavelength such that the absorbance for precise processing at that second wavelength is insufficient but still too high for optical application. Then, when laser processing is performed at the first wavelength, the material may now be highly transparent after material conversion for the second wavelength and may now be moderately absorbent for the first wavelength. That is, in the material conversion, a certain transmittance (ideally up to total transparency) in the processing wavelength range depending on the processing wavelength is achieved. In any case, for the initial state of the component sufficient absorption of the processing wavelength for a precise processing and the final state sufficient transparency for the desired application wavelength (s) to fulfill the optical function prevail.

As a material for the absorption layer SiO _{x is} particularly suitable for use with UV lasers. The SiO _x material with 1 <x <2 is a non-stoichiometric silicon oxide compound (partially oxidized, but macroscopically homogeneous material), which is highly absorbing for UV radiation. As SiO ₂ (fully oxidized material), however, the material is highly transparent to UV radiation and also has a high damage threshold so that it is suitable for high energy densities. The material conversion is realized by heating the material under an oxidizing atmosphere (for example in air) in a suitable device. In this case, the SiO _x material is oxidized to SiO ₂ . In experimental series, thermal material conversion (oxidation) of an SiO _x component in air has proven to be particularly effective for about eight to nine hours at about 900 ° C. Through this treatment, a particularly high transparency (intrinsic transparency> 90% for 193nm) can be achieved. Shorter oxidation times and / or lower temperatures give inferior resulting transparency values, with longer oxidation times and / or higher temperatures, no significant further improvement can be achieved. In principle, a photochemical oxidation by surface or locally resolved laser irradiation below the ablation threshold in an oxidizing atmosphere with or without further thermal treatment is possible. It is also conceivable to convert material in a locally resolved manner, which results in even more extensive possibilities of producing optical structures (also independent of material removal).

In principle, the method can also be used with other materials in the visible or even in the infrared spectral range. As materials, oxidic materials such as metal oxides and semiconductor oxides can be used in principle. In addition to silicon oxide (SiO _x ), aluminum oxide, scandium oxide, hafnium oxide and yttrium oxide are particularly suitable for the UV wavelength range. Tantalum oxide and titanium oxide are particularly suitable for the visible spectral range.

Because a processing cycle consisting of a layer deposition and a structure-producing ablation is performed several times, a multi-stage component with more than two height levels can be produced. This is particularly advantageous in the production of diffractive phase elements (DPEs), since multilevel (eg 4-, 8-, 16-stage) DPEs have a higher diffraction efficiency. Since with appropriate adjustment of the laser energy density, the number of pulses and the layer thickness, an ablation over the entire depth of the ablation is adjustable down to the substrate, the respective interface between the substrate and the absorption layer can be used as a "predetermined breaking point". This interface makes it easier for a substrate in a defined range of a layer evenly and clean (with high surface quality). Thus, in comparison to the processing of a solid body or in the case where the absorption layer is removed only over part of the layer thickness, an even higher accuracy (with respect to the step height) can be realized in patterning laser processing. In a Vorderseitenablation (irradiation directly on the absorption layer) can be prepared in this way with n cycles a 2 ⁿ -stufiges element. As in a Rückseitenablation (irradiation by the substrate) is removed correspondingly in each case the entire (until then built) layer system down to the substrate, one can generate here with n exposures an n + 1-stage element. For more complex structures, combinations of anterior and posterior ablation are also conceivable. In principle, however, it is also conceivable to generate targeted gradations within an absorption layer by, controlled by the irradiation energy, at different locations only a certain portion of the layer thickness is removed. Deposition and ablation can be performed on different devices, but the laser ablation process can also be integrated into a deposition apparatus.

Further details of the invention will become apparent from the following detailed description and the accompanying drawings, in which a preferred embodiment of the invention is illustrated, for example. The FIG. 1 Figure 4 shows a schematic for making a four-stage diffractive phase element by multiple layer deposition and laser ablation.

A method for producing an optical component 6 is essentially based on multiple passes through a processing cycle consisting of a deposition step of an absorption layer and a structure-forming laser ablation step and on a material conversion in which the component 6 is converted into a final state transparent to the laser radiation.

The method is explained using the example of a four-stage diffractive phase element (DPE) for UV wavelengths. Its mode of action is based on the flexion of Light on a finely structured surface relief in an optical material. By diffraction and interference of an incident electromagnetic wave, for example a laser beam, at the DPE, a desired intensity distribution in a so-called signal plane can be effected. In DPEs, only the phase of the light wave is modulated, ie they allow virtually completely transmissive elements (diffractive amplitude elements (DAE), on the other hand, modulate the amplitude of the incident light wave, ie they are always lossy). For example, this can be used to shape and homogenize the beam profile of an excimer laser for subsequent use. The necessary surface relief is previously calculated by means of a computing algorithm known in principle, for example a computer-generated hologram. The total depth of the structure is given by the relationship D = (q-1) / qxλ / (n-1), where q is the number of height levels, so q-1 is the number of stages, λ is the wavelength at which the DPE its optical function and n is the refractive index of the material of the DPE in air. For a four-step element for an application wavelength of 193 nm, for example, a refractive index of n = 1.561 results in a total structure depth of 258 nm at a respective step height of 86 nm.

On a substrate 1, advantageously formed as a quartz body, by vapor deposition (deposition), a first absorption layer 2 by means of a suitable, known in principle apparatus applied. Via a (not shown) first (calculated) mask the absorption layer 2 is then ablated at the irradiated positions down to the substrate height with a laser radiation 7 ( Fig. 1a ). Alternatively, the editing can also done by pixel-wise driving off the surface of the component 6 via a corresponding control of the laser. The laser energy for ablation is adapted to the applied layer thickness and is chosen so that the absorption layer is completely removed, but the substrate 1 remains undamaged. Processing takes place on the substrate side, ie as a backside ablation. For laser ablation, for example, a UV excimer laser with a wavelength of 193 nm, ie with the same wavelength, which is provided for the later function of the DPE, is used. The absorption layer 2 consists of a SiO _x material which is strongly absorbing at 193 nm. The result of this first processing cycle is a surface having a structure 3 with two height levels 4, 4 ', ie with a level 5 (FIG. Fig. 1b ). So a four-level element has four levels and three levels. A second processing cycle begins with the deposition of a second absorption layer 2 'which is vapor-deposited onto the structure 3 produced in the first cycle ( Fig. 1c ). This is followed by a second ablation with a structuring laser radiation 7 '(via a second mask or pixelwise), the structure 3' having three height levels 4, 4 ', 4 "(FIG. Fig. 1d ). In a third processing cycle of coating (deposition) and ablation, the structure 3 "with four height levels 4, 4 ', 4", 4'"(FIG. 1e) is produced in the same way via an absorption layer 2" with a laser ablation 7 ". In a final thermal material conversion ( Fig. 1f ), in which the component 6 is heated in an oven for several hours in air at several 100 ° C, oxidized the structural material SiO _x to SiO ₂ and thereby becomes transparent to the laser wavelength. Thus, finally, from the structure 3 ", a four-level transparent profile 8, the fulfills the desired optical function as DPE for the operating wavelength.

If, instead of the multiple ablation and deposition of an absorption layer, a solid body is processed or not the entire layer thickness removed, the pulse energy density and the pulse number of the laser are utilized to achieve defined step depths, or a quasi-continuous profile, in the absorbing material, where Here, a very precise setting of the above parameters and the beam profile (laser beam characteristic), possibly with the aid of upstream optical elements arrives to achieve high accuracy, since the (helpful) "break point" substrate - absorption layer is eliminated.

Claims (9)

Method for producing an optical component with which an optical function for electromagnetic radiation of an application wavelength range can be achieved due to its surface structured by a multistage profile, wherein the surface structuring is carried out with the aid of ablative laser processing with laser radiation of a processing wavelength range,
characterized,
in that, in order to produce the multistage profile (8), a processing cycle is performed several times, each consisting of a deposition step and an ablation step, wherein - In the deposition step, an absorption layer (2, 2 ', 2 ") of an oxide material in an incompletely oxidized raw state, the laser radiation of the processing wavelength range in sufficient for Ablatierende laser processing and the electromagnetic radiation of the application wavelength range in absorbs the optical function to a high degree and has a lower level of absorption for the electromagnetic radiation of the application wavelength range in a completely oxidized state, is applied to a substrate body (1) transparent to the processing wavelength range, and - In the ablation step, the applied absorption layer (2, 2 ', 2 ") is irradiated with laser light of the processing wavelength and ablated at the irradiated points at least over a portion of the layer thickness, and in that at least one oxidation conversion step is performed, wherein the generated profile (8) is converted to a final state in which it is transparent to the electromagnetic radiation (7) in the application wavelength region to a degree sufficient for the optical function. Method according to claim 1,
characterized in that
after each processing cycle or after selected individual processing cycles, the material conversion step for the respective absorption layer (2, 2 ', 2 ") is performed. Method according to one of the preceding claims,
characterized in that
the processing cycles comprise front ablation steps in which the absorption layer is directly irradiated and / or has back ablation steps in which the absorption layer is irradiated by the substrate body (1). Method according to one of the preceding claims,
characterized in that
as a raw material a non-stoichiometric SiO _x compound with an average of 1 <x <2 is used, and
that the SiO _x material is converted by the material conversion into a final state of SiO ₂ . Method according to one of claims 1 to 3,
characterized in that
the raw material of a material group consisting of Alumina, scandium oxide, hafnium oxide, yttria, tantalum oxide and titanium oxide. Method according to one of the preceding claims,
characterized in that
the material conversion is an oxidation step by thermal treatment of the component in an oxidizing atmosphere. Method according to claim 6,
characterized in that
during thermal oxidation, the component is exposed to a temperature of about 900 ° C for eight to nine hours. Method according to one of the preceding claims,
characterized in that
by irradiation of the component with a laser radiation in an oxidation atmosphere, the irradiated material is photochemically converted, at least in some areas. Method according to one of the preceding claims,
characterized in that
the processing of the component takes place by a pixel-by-pixel irradiation of the processing surface in successive steps or that the processing takes place over the whole area with the aid of at least one imaging element.

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